The present invention relates to a mobile radio communications system which realizes the utilization of public communication networks with mobile stations.
Conventionally, the personal handy phone system (PHS) regarded as a type of mobile radio communications systems has a problem that a cell, i.e., a radio zone covered by a base station is so small that a handover operation for switching from one cell to another is frequently performed during the high speed movement of a user making a call with a mobile station, whereby the call is temporarily interrupted for several seconds every time the handover operation is performed, thus largely affecting the degradation of communication quality.
Communications systems for the personal handy phone system have been described in an article xe2x80x9cTrends of Digital Cordless Telephonesxe2x80x9d (Transactions of the Institute of Electronics, Information and Communication Engineers, Vol. 77, No. 6, pp 636-642) about a concept of technical specifications for the systems.
In a personal handy phone system for public communications, each mobile station, when moving from one radio zone to another, automatically re-initiates a call in the destination radio zone, thus autonomously realizing the handover operation only by a normal call operation, without any particular intervening handover related operation from the network side. The first edition of the second-generation cordless telephone system standard (RCR STD-28) published by Radio Wave System Development Center Foundation (RCR) defines that the personal handy phone systems for public communications shall employ the above-mentioned re-call handover operation scheme.
The employment of the re-call handover operation scheme enables mobile stations to switch a radio zone only with a normal call operation without any particular operational intervention from the network side, thus providing a simplified handover operation. However, a mobile station needs to re-initiate a call in a destination radio zone every time it performs a handover operation as it is moving from a radio zone to another, so that the communication is interrupted during a handover operation for a time period required for the re-initiation which may delay several seconds. Since a radio zone coverage is quite small in the personal handy phone system, if a call is being made while moving at a speed higher than a slow speed of a vehicle, the handover operation will be repeated so frequently that interruptions of a call, caused by the handover operation, will occur at short intervals to an innegligible degree.
Assume for example that a mobile station is moving at 60 kilometers per hour (km/h), and a radio zone covered by a base station has a diameter of approximately 100 meters (m). A simple calculation based on these assumed values shows that the handover operation is performed every six seconds, and a call is interrupted for several seconds at each handover operation, thus presenting a problem that a precise communication quality cannot be ensured.
FIG. 1 illustrates an example of the configuration of a conventional personal handy phone system for showing how the position of a communication slot transits on signal lines (500) and on radio transmission channels (300a)-(300c) when the handover operation is performed each time a mobile station (100) moves from a radio zone to an adjacent radio zone.
In the conventional personal handy phone system as illustrated, when the mobile station (100) under communication is moving from a radio zone (200a) to an adjacent radio zone (200b) and further to another radio zone (200c) covered by respective base stations (400a)-(400c) in succession, the mobile station (100) re-initiates a call to a mobile communications exchanger (600) through a corresponding base station (400a)-(400c) forming the radio zone (200a)-(200c), each time the mobile station (100) enters a new radio zone (200a)-(200c), to carry out the handover operation. Since a call is again set each time the mobile station (100) moves from the radio zone (200a) to (200b) and from (200b) to (200c), the position of the communication slot for this call is different on the signal lines (500) and on the radio transmission channels (300a)-(300c) in the respective base stations (400a)-(400c).
A technique for switching a pertinent radio zone without performing the handover operation as a mobile station moves from a radio zone to another may be represented by a communications system adapted by the Japanese Shinkansen for its transportation service and public telephone service. However, since the network side detects movements of respective trains from one radio zone to next and realizes the switching of the radio zones by the control performed on the network side, a movement detecting means and a radio zone switching control means are required on the network side. While the provision of these means is effective in a system having a relatively small number of base stations, a system employing a concentrated control scheme for the radio zone switching, or the like, it is difficult to apply the same to the personal handy phone system which includes a large number of base stations, each of which autonomously performs the radio zone switching.
The present invention has been made in view of the problems mentioned above, and its principal object is to reduce the number of times the handover operation is performed in a fast moving mobile station to reduce the occurrence of call interruptions caused by the handover operation, thus ensuring a precise communication quality for such a fast moving mobile station in a mobile radio communications system.
To achieve the above object, the present invention provides a mobile radio communications system having a function of autonomously switching a base station connected to a fast moving mobile station under communication among base stations installed along a moving route of the mobile station in succession without performing an handover operation, thus making it possible to reduce the frequency of occurrence of the handover operation in the fast moving mobile station under communication.
Means for solving the problem of the present invention are described in detail as follows.
According to the present invention, there is provided a mobile radio communications system having a plurality of base stations accommodating a plurality of mobile station through radio transmission channels, each mobile station connected with a mobile communications exchanger through a plurality of signal lines, each of the signal lines parallelly connected to the plurality of base stations, each base station including a mechanism of selecting a particular signal line from the plurality of signal lines and allocating a communication slot for a mobile station dominated by the base station at a particular slot position on the selected signal line, and a mechanism of allocating a communication slot for the mobile station dominated by the base station at a particular slot position of a radio transmission channel at a particular frequency, wherein each base station autonomously allocates a frequency of a radio transmission channel and a time slot on the radio transmission channel used by a mobile station dominated by the base station, when the mobile station was dominated by an adjacent base station, for the mobile station dominated by the base station, in accordance with a lapse of a fixed time period calculated from a previously assumed moving speed of a moving mobile station under communication, each base station allocates a signal line and a communication slot on the signal line used by the mobile station dominated by the base station, when the mobile station was dominated by the adjacent base station, for the mobile station dominated by the base station, and the moving mobile station autonomously switches a base station to which the moving mobile station is connected, successively among the plurality of base stations installed along a moving direction of the moving mobile station under communication, without intervention of operations on the network side and without performing a handover operation.
According to the present invention, there is provided the mobile radio communications system characterized in that each base station further comprises radio signal delay control means capable of variably setting a delay time for a transmission timing when each base station transmits a signal to a mobile station accommodated in an associated radio zone associated with the base station, so that when the respective base stations transmit digital modulated signals to a mobile station, the respective base stations mutually match the phases of their time-division multiplexed channel slots allocated for time-division multiplexed digital modulated signals sent from the respective base stations onto associated radio transmission channels and transmitted to the mobile station.
According to the configuration of the present invention described above, a base station connected to a fast moving mobile station under communication is autonomously switched among base stations installed along a moving route of the mobile station in succession without performing an handover operation to suppress the occurrence of the handover operation in the fast moving mobile station under communication, thereby making it possible to reduce the occurrence of call interruptions caused by the handover operation and to ensure a precise communication quality for the fast moving mobile station in the mobile radio communications system.